If you’re thinking about joining up with Sprint, you better be sure you’re willing to stick with the contract. An internal Sprint document has been leaked that states the fee for early terminations is going up on September 9. It’s not a modest increase either.

Sprints ETF is increasing from $200 to $350 for advanced devices. Advanced devices include smartphones, tablets, netbooks and notebooks.

The good news is this new early termination fee is not for existing customers. Only contracts signed on September 9 or later will be subject to the new ETF.

By comparison, AT&T’s ETF is “$325 minus $10 for each full month of your Service Commitment that you complete.” Verizon has a $350 ETF and T-Mobile is the cheapest of the bunch with a $200 ETF.